Method of molding hollow articles from refractory materials under heat and pressure



Dec. 26, 1950 G WATSON 2,535,180

METHOD OF MOLDING EQLLOW ARTICLES FROM REFRACTORY MATERIALS UNDER HEAT AND PRESSURE Filed Feb. 5, 1947 3 Sheets-Sheet 1 Fig. 1

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' ,47 INVENTOR.

Q \M GEORGE RWAmoN v BY Dec. 26, 1950 s. R. WATSON METHOD OF HOLDING HOLLOW ARTICLES FROM REFRACTORY MATERIALS UNDER HEAT AND PRESSURE Filed Feb. 5, 1947 3 Sheets-Sheet 2 Fig.16

.mmnon. GEORGE RWAWQN m I '7 Dec.-26, 1950 G R w TsoN 2,535,180

METHOD OF MOLDING EOLLOW ARTICLES FROM REFRACTORY MATERIALS UNDER HEAT AND PRESSURE 3 Sheets-Sheet 3 Filed Feb. 5, 1947 INVENTOR GEORGE R. WATSON WHM ATTORNE Y Patented Dec. 26, 1950 METHOD OF MOLDING HOLLOW ARTICLES FROM REFRACTORY MATERIALS UNDER HEAT AND PRESSURE George R. Watson, Chippawa, Ontario, Canada,

assignor, by mesne assignments, to the United States of America as represented by the United States Atomic Energy Commission Application February 5, 1947, Serial No. 726,469

Claims.

The invention relates to a method of molding hollow refractory materials under heat and pressure.

One object of the invention is to mold thin walled hollow pieces of refractory material under heat and pressure. Another object is successfully to mold hollow pieces of refractory material which has a coeflicient of expansion materially higher than graphite. Anotherobject is The invention accordingly consists in the several steps and relation and order of each of said steps to one or more of the others thereof, all as will be illustratively described herein, and the scope of the application of which will be indicated in the following claims.

In the accompanying drawings illustrating several applications of the method of this invention,

Figure 1 is an axial sectional view of a mold,

successfully to mold hollow pieces of refractory plungers nd cor rod which may be used in materll'ital which has a tendency to stick to arryin out the invention, grail i Figure 2 is a view similar to Figure 1 showing Another object of the invention is to increase a ste in the method, produflltlon and Obtaln Sma r number 0f Figure 3 is an axial sectional view, to illustrate defectlve pieces in the hot pressure molding of th mbodi ent; of the invention, ow pi from trefractoryt materials i Figure 4 is a view similar to Figure 3 showing graphi e mo s. Ano her objec is produce a Step in t t better finishes t smoother bores in such mold- Figure 5 is an axial sectional view, to illustrate ing. Another obect is to provide a method of another embodiment of t invention, hot pressure molding of hollow pieces made of 2 Figure 6 is a view simiiar t Figure 5 howing various oxides such as beryllia, magnesia, a t in t method, alumina, zirconia, titania, calcium oxide, vana- Figure 7 is an end View of the mold assembly giufm oxide, icsltrontgumthoxide,o bai'lum toxide artlld of Figures 5 and 6,

a mum OX 15 provl 9 Figure 8 is a fragmentary axial sectional view hot Pressure molding of Pieces made of of a mold assembly to illustrate how the method various other refractory materials such as'boron may be applied to muitipie pressure molding, carbide, titanium carbide, titanium nitride, and Figure 9 is an end view of t moveabie fur- Zirconium carbidenace plunger of Figure 8 on a reduced scale,

Another object is to achieve some or all of the Figure 0 is an end View of t mold body of above obiects in the hot pressure molding a Figure 3, on a reduced Scale, number of Pieces in a single Operation, using any Figure 11 is an end view of the stationary furof the materials mentioned above or others. Anmace plunger of Figure 3 on a reduced scale, other oblect is to Provide a satlsfactoly method Figure 12 is a side elevation of a, portion of the g g gfggi ig f gg g 2 z A piston rod 240 of the aforesaid patent to Ridgway e e ne an No. 2,125,588 with a plain ram head thereon, other oblect is to provlde a Satisfactow method Figure 13 is a side elevation of a portion of the and apparel? Pressure mOldlng Wt 01 screw shaft 224 of the aforesaid patent to Ridgmatenals of hollow pieces having bores way No. 2,125,588 with a plain ram head thereon, whlch are not cylindrical, such as tapered bores, Figure 14 is an axial sectional View of a mold. gz ggggg g bores having ribs or other plungers and core rod for molding crucibles acding to the invention A general ob ect of the invention is to increase our the versatility and usefulness of hot pressure i g; i g z figg to Figure 14 Show molding. l Another object is to provide improved i i f a 1d 1 q a d equipment and better methods for molding with gum s a V ew 8 apparatus constructed along the lines of core rod of Figures 15 and 16, but on an enlarged P t t to R Ridgway, 2,125,588 Another scale. showing a further step in the method, and object is to provide im roved method for mold shoving t e crucible completely fOrmBd but ing with graphite molds. Another object of the in t mold, invention is to provide an improved method of Flgure 17 is an axial sectional view of a sp cial molding mixtures of refractory materials to mold p to od ram head and Conne t n l nk to co which has been a problem. meet the piston rod to an e ector rod to withdraw Other objects will be in part obvious or in part the core rod of Figures 14, 15 and 16, pointed out hereinafter. Figure 18 is a view partly in axial section and partly in elevation of apparatus according to U. S. patent to Ridwa 2,125,588 which can be v used in practicing the present invention.

The method of the present invention may be carried out with many types of furnaces equipped with pressure apparatus. For example high frequency induction furnaces may be used. How ever the most practical and readily available high temperature pressure furnace now known to me is the furnace described in the aforementioned Patent No. 2,125,588 to R. R. Ridgway, and therefore the invention will be described as it is used in conjunction and association with that fur- 118.06.

Referring now to Figure 18, the furnace of the aforementioned Ridgway patent has a pair of standards I20-I20 which may be similar but oppositely oriented. The standards I20I20 are conected by a longitudinal frame member I2I. The standards I20 support tubes I21, preferably three in number in which are slidably mounted tubes I28.

Resting on the standards I20 are chairs I30-I30, there being preferably two of these supported by each standard I20. These chairs I30 support aluminum cylinders I 32 and I33 having flanges I34 and I35 which are secured together by bolts, not shown. An insulating ring I31 is interposed between the flanges I34 and I35. Each cylinder I32 and I33 has a rectangular portion I39 forming an opening therein and covers I40 normally close these openings.

Closing the otherwise open end of each cylinder are annular end plates I4I. These annular plates I4I may be made of aluminum or other suitable material and ma be welded to the cylinders I 32 and I33 respectively. By the provision of aluminum cylinders partly closed by aluminum end plates there is provided a nonmagnetic material surrounding the central heating chamber of the furnace, and at the same time the material is electrically conductive. Through the cylinders I32 and I33 passes a heavy heating current of non-inductive characteristics.

The annular plates I M have integral cylindrical portions I43 geometrically projected from the inside bounding circles of the plates. The portions I43 constitute supports for the heating resistance element I50 and the supporting electrodes which convey curent thereto. The resistance element I50 is a graphite tube and current flowing by way of the cylinders I32 and I33 flows through this graphite tub I50. The details of the electrodes for the support of the graphite tube I50 and the water cooling thereof will be found fully described in the aforesaid Patent 2,125,588 and need not be repeated herein.

Integral with the flanges I34 and I35 are upwardly extending wings I15 and the insulating ring I31 has a similar upward extension I16. To the wings I15 bus bars I11 and I18 are fastened. The bus bars I11 and I18 are each of them branching bus bars and are of interlaced construction which insures that they shall have low inductive reactance. It will now be seen that current can flow from the bus bars I11 and I18 through the cylinders I32 and I33 and through the graphite tube I50 and that by reason of the geometry of the current path the entire furnace will have low inductive reactance.

There is provided at one end of the furnace movable pressure apparatus and at the other end thereof adjustable apparatus to take the thrust.

Referring to the left hand side of Figure 18 the tubes I 28 support a spider 220 having parallel bores through three bosses 22I on the ends of the three arms thereof and having a central hub 222 in which is a nut 223 secured thereto. Extendin through the nut 223 is a screw shaft 224 on the left hand end of which is fastened a hand wheel 225. The spider 220 is removably secured to the tubes I28 by pins 243 passing through the bosses HI and the tubes I23.

Referring now to the right hand side of Figure 18, a piston rod 240 is connected to a piston 245 in a cylinder 246 which is supported by spiders 241 and 243 having bosses 249 and 250 through which the tubes I23 pass. Longitudinal thrust is transferred from the cylinder 245 and 246 to the tubes I28 through pins 25I similar to the pins 243 and having the same function. The cylinder 246 has cylinder heads 252 and 253, and a pipe 254 connects to the left hand end of the cylinder 246 through the cylinder head 252, while a pipe 255 connects to the right hand end of the cylinder 245 through the head 253. If desired a gauge 256 may be provided conected to the right hand end of the cylinder 246 by means of a pipe 251. Each of the pipes 254 and 255 leads to a triple valve 260 having an operating handle 26I, and a pipe 262 connects by way of a valve 263 to piping 264 leading to a source of air under pressure, steam under pressure, or the like. Triple valves being known, no cross section thereof is shown, but in one position of the handle 26I air is admitted to the right hand side of the piston 245 while the left hand end of the cylinder 246 is connected to an exhaust pipe 265 which simply exhausts into the air. In an opposite position of the valve handle 26I air or steam is directed to the left hand side of the piston 245, and the right hand side of the cylinder 246 is connected to the exhaust 255. In a third, which is a mid or neutral position of the handle 26I, the flow of air or steam is shut off altogether by the valve 230, and both sides of the cylinder 246 are connected to exhaust 265, or th ports connecting to the pipes 254 and 255 may be blocked. By means of the handle 26I the piston 245 may be moved to the right or left, and when moved to the left it may be thrust in that direction with great force.

In the manufacture of molded pieces, without any binder, from various oxides, such as those mentioned, problems have been encountered which were not encountered in the molding of ,boron carbide, B40. One reason is the coefficient of expansion difference which is as in the following table.

TABLE Coemcients of expansion, measurements being transverse to the molding axis. All figures x10 per degree centrigrade.

Good as an Coefllcient Average 051 Ranfge t overt ereo empero ure subsmnm (oil figures ango approxi- Below in mate) De recs Cent grade Graphite 3 7 to 6. 4 Mi-2,000 Boron Carbide, B40 4. 6 20-2, Beryllia 7.8 20-l,800 Magnesia ll. 6 201,280 Alumina 6. 3 20-1,?50 Thoria 10.3 204,400 ZilCO 11 204,600

same range as that of graphite, while the oxides listed (and probably othe s on which I have no good figures) have coefficients of expansion significantly higher than graphite.

The use of graphite cores in the molding of these oxides therefore seemed to be precluded, but graphite was and is the most practical material for cores as well as mold bodies on account of its refractorlness, non-reactivity at the temperatures involved, and its high strength at high temperatures.

Another problem encountered with the use of graphite cores in the manufacture of molded pieces especially of boron carbide is sticking of the material being molded to the core. However I know of no satisfactory substitute for graphite for the molds and cores in this type of hot pressure molding.

Referring now to Figure 1, I provide a mold body 20 which is a cylinder of graphite with a coaxial bore containing an undersized cylindrical mold core 22 also made of graphite. The mold body 20 and the core 22 are in this case of the same length. I provide two cylindrical sleeves 24 whose outside diameters are such that they fit with a sliding fit in the bore of the mold body 20, and whose inside diameters are such that they fit with a sliding fit on the core 22. These sleeves 24 are also made of graphite and in this case they are of the same length.

Referring now to Figures 12 and 13, the piston rod 240 of the Ridgway furnace has on the end thereof a plain ram head 25 and the screw shaft 224 of the Ridgway furnace has on the end thereof a similar ram head 25a. These ram heads 25 and 25a simply consist of plates 26 of steel integrally attached to hollow steel hubs 21 which fit with a loose fit on reduced end portions 28 and 29 of the piston rod 240 and screw shaft 224 respectively. The loose fit is so that strains will not be set up in the graphite plungers in case the end surfaces thereof are not exactly squared. The ram heads 25 and 25a can be picked off of the piston rod 240 and screw shaft 224 at any time that these are withdrawn.

Referring now to Figure 1, I provide graphite plungers 30, 3|. These plungers are directly contacted by the ram heads 25 and 25a. These graphite plungers 3'! 3| are cylindrical and they fit with a comfortably loose fit in the graphite heating tube |50 of th Ridgway furnace. They are of the same outside diameter as the mold 20 and they have axial bores which have tl-e same diameter as the inside diameter of the sleeves 24. In the bore of the plunger 33 is a graphite rod 32 which has a diameter somewhat less than the bore in which it fits, so that there is substantial clearance. Th length of the rod 32 is, in this case, the same as that of the plunger 33.

Before the mold 20 is placed in the graphite tube I50 of the Ridgway furnace, it is filled with material 34in the form of a powder. This may be any of the carbide or nitride powders mentioned, or powder of any of the oxides mentioned, or mixtures of powders, or any other refractory powder which can be hot pressed to form an integral body. A satisfactory manner of filling the mold is, first to place the core 22 part way in a sleeve 24, then insert core and sleeve in the mold body 20 so that th ends of the core 22 are fiush with the ends of the mold body 20, place the mold parts with their axes vertical, fill the space between the body 20 and the core 22 and insert the other sleeve 24. The assembly of mold 20, core 22, sleeves 24 and powder 34 is then taken to a hand press operating on the sleeves 24 to compact the powder as much as possible while it is cold, and the parts are adjusted so that each sleeve 24 extends to the same distance respectively both into and outside of the mold. Then the assembly is placed in the graphite tube I50 of the Ridgway furnace.

At some stage the plungers 30 and 3| are inserted in the furnace tube. The plunger 30 having the rod 32 should be at the live or piston rod end, while the plunger 3| having no rod therein should be at the "dead or screw shaft end. Also the ram heads 25 and 250. are adjusted into contacts with the plungers, by turning the screw shaft 224 and manipulating the air valve 260 of the Ridgway furnace. The adjustments should be so made that there is continuous contact from ram head 25 to plunger 30, to sleeve 24' on one side of the mold, and from sleeve 24 on the other side to plunger 3| and ram head 25a, and the mold body 2|) should be so located in the furnace tube that it will be at the center thereof at the peak of the operation. In connection with other embodiments of the invention to be described it should be understood that the molds,

may be filled and cold compacted in similar fashion and the plungers and rams may be adjusted in the furnace by the above or any equivalent procedure, the Ridgway furnace having many provisions for assembly and adjustment. It may be noted that in this and other embodiments of the invention, the graphite plungers extend well out of the furnace tube so that the power ram 25 On the piston 240 can move a substantial distance driving its plunger inwards, without contacting the furnace tube.

The Ridgway furnace is now ready to operate and is operated as described in the Ridgway patent. This involves heating of the furnace tube I50 of the patent by the flow of electric current therethrough, and the application at the same time of pressure by the ram head 25 on the plunger 30, the thrust against the mold being taken by the plunger 3| and the other or stationary ram head 25a. Since the mold 20 is free to move axially in the furnace tube both of the sleeves 24 are driven inwardly and compress the heated material 34 which sinters to form a solid sleeve of uniform structure and density clos to the actual density of the material. In other words, the porosity of the article made from the material 34 will be low.

When, by the movement of the piston rod 240 and the time elapsed, the temperature reached and the pressure exerted, the operator knows that the heat may be turned off he performs the special step which is a particular Lature of the present invention. This may be done without turning off the heating current, or so soon after turning off the heating current that the material 34 has not had a chance to cool much. The piston rod 243 is caused to retreat a considerable distance by means of the air valve 233 of the Ridgway furnace and then a short rod 35 of graphite (see Figure 2) having the same diameter as the rod 32 is placed against the rod 32 in the bore of the plunger 30, the ram head 25 of the piston rod 243 is new advanced to contact and then pressure is slowly applied. This forces the graphite core 22 right out of the hot material 34 and along the sleeve 24 part way into the bore in the plunger 3|, as clearly shown in Figure 2. The heating current is now shut off or if it was already shut off, it is not again turned on. The pressure is not reapplied. The

material 34 may now cool and shrink and will not be cracked by the core 22 having a lower coeflicient of expansion nor will it stick to said core. Neither will it be cracked by the graphite rod 32 since the rod is undersized, that is of substantially smaller diameter than that of the core 22. The material 34 will not stick to the rod 32 because it is not in firm contact therewith.

when the furnace has cooled considarably, the plungers and mold assembly are removed from the furnace. The molded piece 34 is now taken out of the mold 2 0. In the case of some materials and some shapes the piece 34 comes free from the bore of the-mold 20, while in other cases the mold is cut off of the piece 34. The core 22 however is not in the piece '34 at all and the rod 32 readily slips outof it.

Referring now .to Figures 3 and 4, another embodiment of the invention is disclosed for the molding of such materials as develop so much friction between themselves and a graphite core during heating that it is difficult to remove the core in one step as above described. In this embodiment there are a cylindrical mold body 20a, a core 22a, two cylindrical sleeves 24, and graphite plungers and 3|. All of these mold parts and the plungers are made of graphite. I further provide a graphite rod 32a which is relatively longer than the rod 32 and extends from the outer face of the plunger 30 to the core 22a. Conscquently, during the sintering operation, while the heat and pressure are on and the material 34a is being compacted, the core 22a is gradually moved tothe left arriving about at the position shown in Figure 4 when the material 34a has become fully compacted. With some materials this procedure prevents sticking which would otherwise occur. since adhesion forces do not have a chance to build up, being destroyed regularly. When the sintering under pressure is completed, the core 22a is still fully within the material 34a. The core 22a may now be removed by providing a short rod 35 (Figures 1 and 2) and proceeding as in the case of the first embodiment described, the core 22a going part way into the bore in the plunger 3|. The remainder of the procedure is the same as first described and it is noted that the rod 32a should be undersized, that is of substantially less diameter than that of the core 22a.

Another advantage of this (second) embodiment of the invention is that the end of the rod 32a which finally enters the bore of the material 34a has been preheated to a greater extent than in the case of the other (first) embodiment of the invention, thus avoiding heat shocks to the material being molded, which heat shocks have in the case of some materials and some shapes caused cracks.

The above procedure and apparatus of this (second) embodiment is particularly useful for the oxides. However for the carbides the following (third) embodiment is preferred. Particularly boron carbide and titanium carbide develop so much adhesion to hot graphite in such a short time that for certain sizes and shapes intermittent moving of the core can not be successfully accomplished.

Referring now to Figures 5, 6 and '7, I provide a cylindrical mold body 28b of graphite having an axial bore, two sleeves 24b of graphite and graphite plungers similar to the plungers 39 and 3| of Figures 1 to 4. I further provide a thin hollow cylindrical graphite core 40 which fits with a sliding fit on a solid cylindrical graphite core 4|. This mold is filled with molding material 34b which, in Figures 5 and 6 has been compacted. While the material 3417 is still hot the solid graphite core 4| is pushed out of the thin hollow graphite core 40 by the same procedure as described in connection with Figures 1 to 4, using a rod 35. Figure 6 shows the solid core 4| removed. The core 40, or what remains of it, can be readily cut out of the piece 34b or sandblasted away. In either case it is an easy job compared with the removal of a solid core. Furthermore all danger of cracking the molded piece 34!) is eliminated, since the thin core 40 will crack, crumble or deform if the piece 34b contracts faster than does the core 43.

Referring now to Figures 8, 9, 10 and 11, the method of the invention is therein illustrated in connection with multiple pressure molding. U. S. Letters Patent No. 2,150,884 to the same R. R. Ridgway shows multiple pressure molding in the aforesaid Ridgway furnace. in which graphite mold bodies with a plurality of bores are used in order to make a plurality of articles during each run of the furnace. I provide a mold body of graphite having, as shown in Figure 10, a plurality of bores whose centers lie on a circle 41 which is coaxial with the mold body 45, the bores 45 being symmetrically spaced on the circle 41. I further provide a live furnace plunger 48 of graphite and a dead end furnace plunger 49 of graphite. These plungers 48 and 49 are preferably cylinders and they have bores 5| and 52, equal in number to the bores 45 and also symmetrically spaced on the (imaginary) circle 41 coaxial with the bodies. The plungers '48 and 49 and the mold body 45 can therefore be lined up in the furnace tube so that the bores 5|, 4S and 52 will be in axial alignment.

I provide graphite mold cores 55, one for each of the bores 4d and of about the same length but of lesser diameter. I provide sleeves 56 of graphite which fit with a sliding fit on the cores and in the bores 46, one for each end of each bore 46 which act as mold plungers like the sleeves 24 and 24b. I provide aligning pins 58 of graphite, one for each bore 52, each pin 58 having a portion 59 which fits the bore 52, and a portion on which fits inside the sleeve 55. I further provide push out plungers 6| made of graphite which fit with a loose fit in the bores 5|. It will be noted that the bores 5| are of less diameter than the bores 52, and the bores 52 are of less diameter than the bores 46.

The mold body 45 is filled with material 62 which may be any of those mentioned or other material, and the mold plunger sleeves 56 are inserted. The dead end furnace plunger 49 is now aligned with the mold body 45 by placing a portion 59 of a pin 58 in each bore 52 and then inserting a portion 60 in each sleeve 5) at one end of the assembly. The live furnace plunger 48 is likewise aligned with the mold body 45 by inserting the push rods 6| part way into the sleeves 55. After the one plunger and the mold body 45 are aligned, the assembly may be pushed part way into the furnace tube and then the other alignment may be made and then the entire assembly may be centered in the tube and the rams 25 and 25a brought into position against the ends of the plungers 48 and 49. The furnace is now ready to operate.

Figure 8 shows a fragmentary view of one bore 46 of the mold body 45 showing the material 52 compacted at the peak of the furnace run,

- 9 i. e. after the material 62 has been heated hot enough and long enough to effect, with the aid of the pressure used, the desired amount of sintering and compacting. The ram head 25 is now withdrawn and short graphite rods like the rods 36 are now inserted in the bores II and the ram head 25 is again advanced to push out the core rods 55. Or a continuous pushing and later ejection of the core rods 85 may be achieved by making the push out rods SI of the proper length. Also in this mult ple pressure molding the feature of using thin walled hollow cores backed up by solid cores may be used as was described in connection with Figures 5, 6 and 7. The cores in any case eventually contact the pins 58 and further movement forces the pins El well into the bores 52 and finally the core rods I! enter the bores 52 which ha e been aligned to receive th reby the core rods I5.

Referring now to Figures l4, l5 and 16, for the mold ng of crucibles or the like I provide a cylindrical graphite mold body 10 having an axial bore in one end of which is fitted a cylindrical graphite plunger 12 and in the other end of which is fitted a gra h te plun er sleeve I3. Fitting with a sliding fit in the sleeve 13 is athin wa led graphite sleeve 14. A long graphite rod I! is closely fitted to the inside of the thin walled sleeve I4 and a shoulder 18 thereof contacts the end of the sleeve '14. The outer end of the rod is bored and threaded to form internal screw threads 11.

This mo d of Figure 14 may be filled in the manner alreadv described, that is to say the rod 15 is inserted n the sleeve 14. then the rod 15 and s'eeve 14 are inserted in the sleeve II and then t e slee e 13 is inserted in the bore of the mol bodv 10. these parts then occupying the r lative position s own in Figure 14. The material M to be mo ded is then poured into the s ace bet een the mold body Ill and the sleeve 14 and coverin the end of the rod 1!, whereu on the mo d plun er 12 is insert d as shown and the assembly s taken to a hand press and t e material an is cold compacted. At the dead end of t efurnace I nrovide a solid graphite furna e lun er al. The dead end plunger BI is ace in t e furnace tube 80 of the Ridgway furnace and is hacked up by the head 28a on the screw s aft 724. I provide a live end furnace p n er 83 which has a bore 85. The plunger 83 is now n'ace over the rod 1! and in contact with the slee e 13. and these parts and the mold 1" are now placed in the furnace tube with th un er 12 in contact with the plunger 8|. It wl he seen that the relative lengths of the rod 15 and plunger 83 are such that the former now proiects beyond the latter at the ri ht. Figure 14. I further provide a short grahite plun er 86 with a bore 01 which I place over the end of the rod 15 and in contact with the lun er 83. This plunger 88 extends beyond the rod I5.

Referring to Figure 17, in this embodiment of t e invention I provide a ram head 90 which has a plate 9| integrally attached to a hub 92 w th a ta ered hole 93 therethrough and I provide a taoer pin 94 passing through an oversized hole 95 in the piston rod il la as well as through the tanered hole 93. I further provide a bore il'i in the plate 9| with a bayonet slot 91 therein.

The ram head 90 is now advanced into contact with the short graphite plunger 86 by manipulating the air valve 260 of the Ridgway furnace. The current is then turned on to heat the for.

10 nace tube I50 of the Rdgway furnace, and the air valve 260 is again turned to apply the pressure. Thus the sintering under pressure starts, and the cylindrical portion of the material around the sleeve 14 is compressed and sintered first.

Although the ram is the live ram and alone does the moving, and the ram 25a is adjustable but stationary during a heating cycle, force is exerted nearly un formly on the two ends of the mold assembly since the latter is freely movable in the furnace tube I50 of the Ridgway furnace. However the force exerted by the sleeve I3 is over a much smaller area than the same force exerted by the plunger 12. Consequently, since the material 80 does not obey Pascal's law, there is greater pressure on the portion of the material around the thin sleeve 14. It is found that, after the material 80 has been heated up, the sleeve 13 will gradually move along the sleeve 14 to the left, but the p unger 12 will not come much, if any, nearer to the rod I5.

Accordingly, after the material 80 has reached its s ntering temperature, the molding pressure having been applied during the heating, the mold assembly is in the condition illustrated by Figure 15. This shows the wall portion of the material 80 as having been compacted, while the portion which will form the bottom of the crucible has not been compacted. The ram head 90 is now caused to retreat and then the short graphite plunger 86 is picked off. Now the ram head 90 is again advanced and the full force is reapplied but this time it is exerted against the rod 15. Now a much greater pressure is developed in the material 80 which will form the bottom of the crucible and it is rather quickly compacted because the material is already at the sintering temperature. The plunger I2 appears to move further into the mo d Ill, but actually the rod 15. sleeve H, sleeve 13 and mold 10 all move to the left while'the plungers I2 and 8| remain stationary. Figure 16 shows the assembly at the end of the sintering and pressing with the crucible 80 fully compacted.

Before the crucible 80 has had a chance to contract by cooling, the rod 15 is removed from the sleeve 14. Referring now to Figure 1'7 I provide a metal screw I00 having threads which will fit the threaded bore 11. This screw Hill has a reduced end portion iill with a bayonet lock I02 thereon. I now withdraw the ram head 90, quickly screw the screw I00 into the bore 11, advance the ram head 90 gently and enter the bayonet lock H12 in the bayonet slot 91, then turn the screw I00 to lock the parts together, then again cause the ram head 90 to retreat. This draws the rod ll to the right and if the entire mold assembly starts to move I take a bar and thrust upon the plunger 83 to hold the mold assembly in the furnace tube 50. Thereby the rod 15 is withdrawn from the sleeve 14, and now the furnace can be allowed to cool with the mold assembly in the heating zone thereof.

In cooling the crucible can contract against the sleeve 14 which will crack or split. The remains of the sleeve 14 can be cut or sand blasted from the crucible after the other mold parts have been removed or cut away.

The reason why the rod 15 is not forced to the right during molding of the wall of the crucible (molding step Figure 14 to Figure 15) is that the pressure causes the sticky boron carbide to squeeze the thin sleeve 14 which grips the rod 15. However I find that the pressure should be raised to be exerted against the material 80 of thewall.

till the temperature has risen nearly to the maximum.

By proceeding in accordance with the invention as described in connection Figures 14 to 17 strong integral dense crucibles can be molded with few rejections. The method described is the best method now known to me for the manufacture of refractory oxide crucibles. For the manufacture of crucibles out of some of the oxides a solid one piece core rod maybe ,used providedit is withdrawn before the material cools.

In all of the embodiments of the invention a pressure of twenty five hundred pounds per square inch against the material or part thereof is used as the highest pressure during the sintering operation. It is preferable to work up gradually to this top pressure. This has been found to be a satisfactory top pressure for all molding operations encountered. Higher or lower pressures could be used but calculations are simplified by using a standard pressure.

In the embodiment of Figures 14 to 17 inclusive, the complete core is the sleeve 14 and the rod 15. When the rod 15, which is the core rod, is removed, it can be said that the core has been moved or removed. At the same time a part of the core, the sleeve 14, remains, and the sleeve 14 is in one sense a core. In the embodiments of Figures 1 to 11 the cores 22, 22a, 4| and 55 are rods.

It will thus be seen that there has been provided by this invention a method of molding according to which the various objects hereinbefore set forth are successfully achieved. As many possible embodiments may be made of the above invention and as many changes might be made in the embodiments above set forth, it is to be understood that all matter hereinbefore set forth or shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense.

I claim:

1. Method of molding hollow bodies from refractory material comprising filling a mold cavity with a core and with the material to be molded, simultaneously heating and pressing until the material is sintered and compacted and then, before the material has had a chance to cool substantially, removing the core.

2. Method of molding hollow bodies from refractory material comprising filling a mold cavity with a core and'the material to be molded, simultaneously heating and pressing and moving the core until the material is sintered and compacted and then, before the material has had a chance to cool substantially, removing the core.

3. Method of molding hollow bodies from refractory material comprising filling a mold cavity witha solid coreon which is a thin hollow core and with the material to be molded, simultaneously heating and pressing until the material is sintered and compacted and then, before the material has had a chance to cool substantially, removing the solid core.

4. Method of making a crucible or other hollow body having a wall portion and a bottom portion out of refractory material comprising filling a mold cavity with a core and the material to be molded, simultaneously heating and pressing-the wall portion until the material is sintered and compacted, then continuing the heating and pressing the bottom portion until the material is sintered and compacted and then, before the material has had a chance to cool substantially, removing the core.

5. Method of making a crucible or other hollow body having a wall portion and a bottom portion out of refractory material comprising filling a mold cavity with a solid core on which is a thin hollow core and with the material to be molded, simultaneously heating and pressing the wall portion until the material is'sintered and compacted, then continuing the heating and pressing the bottom portion until the material is sintered and compacted and then, before the material has had a chance to cool substantially, removing the solid core.

GEORGE R. WATSON.

REFERENCES CITED The following references are of record in the 

